Studies in inbred guinea pigs have found that sensitization with complete Freund's adjuvant releases into the blood a population of thymic-derived lymphocytes which, in the presence of concanavalin A, suppress the proliferative response of autologous lymph node cells to mitogen. A radioimmunoassay capable of detecting Candida albicans polysaccharide at 20 nanograms/ml has been devised. This assay may provide a rapid diagnostic tool for systemic candidiasis.